1. Field of the Invention
The present invention relates to a switching power supply apparatus and a multi-output type DC/DC converter outputting a plurality of voltages from one input voltage, and more particularly to a DC/DC converter provided with a selection circuit and a priority circuit for switching output currents according to the level of each voltage output.
2. Related Art
There is a DC/DC converter as a power supply apparatus converting a direct-current input voltage into a direct-current voltage of an electric potential different from that of the input voltage to output the converted direct-current voltage. Moreover, there is a circuit shown in FIG. 8 as a multi-output type DC/DC converter outputting a plurality of direct-current voltages having different electric potentials to one direct-current input. Such a multi-output DC/DC converter is disclosed in, for example, JP-A 2005-117886.
The DC/DC converter shown in FIG. 8 turns on and off a main switch SW0 based on a pulse P1 or a pulse P2, which are output from comparators CMP1 and CMP2, respectively, the DC/DC converter selectively turns on any one of change-over switches SW1 and SW2 at a predetermined period based on the output of a frequency divider to apply the output current of an inductor (reactor) L to a rectification & smoothing circuit 16a or 16b in a time division method. Thereby, the DC/DC converter outputs direct-current voltages Vout1 and Vout2, which are converted to voltages having desired levels severally, from a first output terminal OUT1 and a second output terminal OUT2.
Because the multi-output type DC/DC converter as shown in FIG. 8 switches the output current of the inductor L by the time division method, the multi-output type DC/DC converter has the advantages of being capable of decreasing the number of inductors, and of being capable of attaining the miniaturization of the direct-current power supply apparatus. However, because the multi-output type DC/DC converter of FIG. 8 distributes the output current of the inductor L to the first output terminal OUT1 and the second output terminal OUT2 at a predetermined period (fixed), the multi-output type DC/DC converter has a problem of the difficulty of keeping the desired electric potentials if the current ratio of the loads connected to the two output terminals greatly changes.
To put it concretely, for example, there is examined the case where, in a system using a multi-output type DC/DC converter the period of the time division system of which has been designed on the supposition that the ratio of the consumption current of the load connected to the first output terminal OUT1 to the consumption current of the load connected to the second output terminal OUT2 is 2 to 1, the ratio of the consumption current of the load of the first output terminal OUT1 and the consumption current of the load of the second output terminal OUT2 is suddenly reversed to be 1 to 2. In this case, if the period of the time division system is fixed, there is the possibility of producing the situation in which the output voltage Vout2 of the second output terminal OUT2, the load of which has increased, does not reach a desired electric potential, and on the other hand in which the output voltage Vout1 of the first output terminal OUT1, the load of which has decreased, becomes higher than the desired electric potential.
In order to avoid such a situation, the inventors of the present invention filed the invention of providing an output selection circuit generating an on-off control signal of a switching circuit that monitors the outputs of comparison circuits to select the output of earlier one for switching the path of the current flowing through an reactor in a multi-output type DC/DC converter including an output voltage detection unit and the comparison circuit (PWM comparator) to each output (Japanese Patent Application No. 2006-121563). However, it became apparent that it was apprehended that, even if the output selection circuit was configured to select the earlier one of the output signals of a plurality of comparison circuits and to intercept the other output signals, when the output signals from a plurality of comparison circuits were input almost at the same time, it was impossible to intercept the signals input later owing to the delay time in the inside of logic circuits and the like, and that the state in which the drive circuits on the subsequent stages did not operate was thereby caused.